The development of high-efficiency power supplies having a higher power density is a continuing goal in the field of power electronics. A switched-mode power converter is a frequently employed component of a power supply that converts an input voltage waveform into a specified output voltage waveform. There are several types of switched-mode power converters including, for instance, an asymmetrical half-bridge power converter.
A conventional asymmetrical half-bridge power converter typically includes main and complementary switches coupled to a control circuit, first and second input capacitors coupled across the main and complementary switches, respectively, an input/output isolation transformer having a primary winding and a secondary winding, a rectifier, and an output filter. The asymmetrical half-bridge converter generally operates as follows in a steady state.
The main and complementary switches alternately conduct current in a complimentary manner to convert an input DC voltage into an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates therefrom a desired DC voltage that is filtered by the output filter. An output voltage is then provided to a load at an output of the asymmetrical half-bridge converter.
The control circuit monitors the output voltage of the asymmetrical half-bridge converter and adjusts a duty cycle of the main and complementary switches accordingly to ultimately control the output voltage. The control circuit thus provides a mechanism to maintain the output voltage at a relatively consistent level despite relative fluctuations in the input voltage and the load.
The asymmetrical half-bridge converter performs adequately in the steady state. Problems may arise, however, when the asymmetrical half-bridge converter must be turned on or turned off.
One way to turn on the asymmetrical half-bridge converter is to place it directly into the steady state from an off state. Prior to turn-on, the output voltage is zero and there is little or no voltage across the secondary winding of the transformer. Consequently, there is little or no voltage across the primary winding of the transformer. If the main and complementary switches are then switched directly to a steady state duty cycle, a current surge into the transformer, in conjunction with little or no voltage across the transformer, may saturate the transformer. High transient currents resulting therefrom may then cause one or both of the switches to fail.
One way to avoid transient currents capable of causing switch failure is to implement soft-start. Initially, the asymmetrical half-bridge converter is off and the duty cycle of the main switch is at zero. To turn on the asymmetrical half-bridge converter, the duty cycle of the main switch is gradually increased until the steady state duty cycle is reached. The output voltage may thus be gradually increased, thereby avoiding high peak currents.
Since the main and complementary switches are on for complementary periods, however, the complementary switch may initially be on for a large period of time. Energy stored in the second input capacitor may, therefore, rapidly discharge through the complementary switch during the initial switching cycles. The rapid discharge may produce a large pulse of current through the complementary switch, causing it to become damaged.
Slightly different but analogous concerns also arise when the asymmetrical half-bridge converter is turned off. Conventional techniques for turning off the asymmetrical half-bridge converter involve turning off the main switch, leaving the complementary switch on. As a result, charge stored in the second input capacitor discharges through the complementary switch, causing transient currents which may damage the complementary switch.
Accordingly, what is needed in the art is a circuit for reducing transient current through the complementary switch, thereby avoiding damage to the complementary switch during a non-steady state operation of the asymmetric half-bridge converter.